1. Field of the Invention
This invention relates to a system for controlling the amount of fuel supplied to an engine that uses a fuel containing alcohol, more particularly to such a system for controlling the amount of fuel supplied at the time the engine is started.
2. Description of the Prior Art
Engine fuels containing alcohol and the like are being used increasingly as substitutes for gasoline and other more conventional fuels. Some of these new fuels are blends including a conventional fuel. For example, a mixed gasoline-methanol fuel containing 85% methanol has become generally known as M85. Others are used without blending.
The characteristics of these alternative fuels differ from those of conventional fuels in various ways. For example, the stoichiometric air-fuel ratio of methanol is about 6.5:1, compared with about 15:1 for gasoline, and its latent heat of vaporization is about 280 kcal/kg, compared with about 80 kcal/kg for gasoline. These differences are substantial. In particular, the higher heat of vaporization of methanol means that the amount of heat required for fuel vaporization will increase with increasing methanol concentration of the fuel. As a result, the fuel does not vaporize easily when the engine coolant temperature is low and the engine becomes difficult to start.
Japanese Laid-Open Patent Publication No. Sho 56-104131 teaches a system for improving the start performance of an engine that uses a hybrid fuel containing alcohol. In the proposed system, the amount of fuel to be supplied to the engine is determined on the basis of the alcohol concentration of the fuel detected by an alcohol sensor and when the engine fails to start when supplied with the so-determined amount of fuel, the amount of fuel is increased or decreased until starting is achieved.
As was pointed out above, an engine using an alcohol-containing fuel is fundamentally hard to start and, therefore, the probability of its starting is not necessarily high even when the fuel supply is adjusted to the optimum amount for starting. In the aforesaid prior art system, therefore, there is a fairly high possibility that the engine will fail to start at the time the amount of fuel supply is optimum for starting the engine. If this should happen, the amount of fuel supply will thereafter deviate further and further from the optimum, making it even more unlikely that the engine will start.
Moreover, if something should go wrong with the system's alcohol sensor, the amount of fuel supplied will be determined on the basis of an alcohol concentration that is different from the actual concentration. As the system is incapable of determining whether the amount of fuel supply should be increased or decreased in such circumstances, the adjustment is likely to be made in the wrong direction. In this case the probability of the engine starting becomes almost nil.
This invention was accomplished in view of the foregoing problems and has as its object to provide a system for controlling the amount of fuel supplied for starting an engine that uses a fuel containing alcohol, which system ensures reliable engine starting even when an alcohol sensor constituting a part of the system malfunctions or breaks down.